1. Field of the Invention
The present invention is generally directed to colored light sources, and more particularly, although not exclusively, to colored light sources used in projectors and/or printers.
2. Description of the Related Art
Colored light sources have been used in different devices. For example, colored light sources have been used in data projectors and in polygon mirror scanning laser beam printers (LBPs).
FIGS. 1 to 3 depict the structure of typical colored data projectors. In FIG. 1, the structure and the light source in a typical DMD (Digital Micromirror Device) based color wheel color-projection system, which can also be referred to as a DLP™ (Digital Light Processing™) system, are shown. The DMD of FIG. 1 includes a lamp as a light source. Light from the lamp passes through an energy projector, a color wheel, an integrating rod, a lens, a light modulator and finally through a projection lens. It should be noted that in lieu of a color wheel and single lamp, three light sources of red, blue and green can be used.
FIG. 2 depicts the structure and the light source(s) for a typical three-panel color projection system based on a three-LCD (Liquid Crystal Display) panels arrangement. The projection system includes three LCD panels that employ separate polarizers. In addition, the projection system includes power supply circuitry, a PBS array, a UHP lamp and an exhaust fan.
FIG. 3 depicts an alternative arrangement of a three-panel color projection system. Instead of having a single lamp, the three-panel color projection system of FIG. 3 includes three colored light sources, each of a different color, namely red, green and blue. The optical path projector associates a collimator lens and liquid crystal panel with each colored light source. In addition, a compound dichromatic mirror and a projection lens are provided, for projecting an image on a screen.
It should be noted that the application of a compact optical path projector light source does not require a pure, coherent, single wavelength light source, because the human eye cannot typically distinguish 5 nm differences in light wavelengths.
Therefore, the stringent requirements on light source spectral purity posed on many other applications are not needed. Such applications include high power semiconductor lasers sources used in telecom industry applications. The light source for the optical path projector can utilize the output of an incoherent laser arrays, provided that the total output power is large, the total quantum efficiency is high (as a laser opposing to light emitting diodes (LEDs)), and the device size is compact.
Turning to the use of colored light sources in LBPs, FIG. 4 depicts the structure of a typical polygon beam scanning LBP. The LBP includes a laser diode 2001 and a polygon mirror 2002 spinning at high speeds (e.g. 20,000 to 30,000 rpm). The LBP also includes an aspherical laser beam-focusing lens 2003, and a photosensitive drum 2004.
A few functional characteristics are preferred for the light sources used in these two applications. First, a compact light source size is preferred. In this regard, portability is important for data projectors. For LBPs, reducing the total structure size and the individual components sizes is considered to be helpful for increasing products' competence.
In addition to compactness, high output optical power is preferred, especially for the data projector applications. In particular, a large screen projector can typically use several hundred to a couple of thousand lumens of net optical power. Assuming that the averaging lumen/Watt sensitivity of a projected averaged color image is ˜300 Lumen/Watt (optical power) for a peak response of 683 Lumen/Watt at green, and assuming that three individual light sources are used to construct the color image projector, then the output optical power of each individual light source is preferably between several hundred milliwatts to a couple of watts.
High wall-plug efficiency, or the ratio between the net output optical power (Watts) and the total consumed electrical power (Watts), is also a preferred functional characteristic. The power conversion efficiency from electrical power to optical powers is less than a few percent for typical incandescent and fluorescent light sources, especially when only the visible spectrum is considered. For visible light LEDs (light emitting diodes), power conversion efficiency is usually a couple of percentage points. The concern for high wall-plug efficiency is less about the power consumption, and more about the excessive heat generation that usually results from unconverted electrical power. The generated heat not only reduces the lifetime and reliability of the light sources, but also results in bulky and power consuming thermal dissipation mechanisms and components, which increase the size, power consumption, and production costs of devices.
Furthermore, a well-defined and stable beam profile (i.e. spot shape) is a preferred functional characteristic. More particularly, a single color source that can output a single lobe, small diameter, and pure Gaussian profiled spot is preferred. In data projectors, such a beam profile can dramatically reduce the needs for complex lens systems for converting into a uniform collimated light beam for projections. In LBPs, such a beam profile can facilitate uniform and fine exposure of pixels.
Another preferred functional characteristic for light sources used in data projectors and LBPs relates to wavelength range. It should be noted that the wavelength range for data projectors and LBP's is significantly more flexible than typical laser systems. Unlike may laser systems used in communications and metrologies, a variation of wavelength within a couple dozen nanometers is well acceptable for free space optical systems in data projectors and LBPs, and for color perception of the typical human eye. In particular, if a single color light source has its physical wavelength varying ±5 nm (i.e. 10 nm bandwidth), that light source should not have any negative influence on the performance of the data projector or LBP equipment, as long as the output optical power and beam profile (including polarization) are stable.
Yet another preferred functional characteristic is the use of an incoherent light source. In many free space imaging systems, incoherent light sources are preferred over coherent light sources, since coherent light sources may introduce unintended interference artifacts, such as speckles. In addition, coherent light sources require additional components such as diffusers, typically leading to increased manufacturing costs and equipment size.
Common light sources used in data projectors are high power incandescent lamps and inertial gas lamps, as shown in the examples illustrated in FIGS. 1 and 2. However, such lamps have several technological limitations. First, these lamps typically have low wall-plug efficiencies. Moreover, since they are white light sources, the mechanisms to filter out RGB (red-green-blue) lights respectively and to dump useless IR (infrared) and UV (ultraviolet) components further reduce the net wall-plug efficiency. This low efficiency not only consumes more power directly, but also generates additional heat, requiring additional components to dissipate the heat. These thermal dissipation components (such as electrical fans and TEC (thermal electrical coolers, or Peltier coolers, with a description available at http://www.digit-life.com/articles/peltiercoolers/)) add cost, size, and power consumptions to the total system.
An alternative to using lamps in data projectors is to use semiconductor LEDs (light emitting diodes). Compared with lamps, LEDs are seen to provide a more compact device size and a higher wall-plug efficiency, as well as a longer device lifetime. Typical characteristics of these LEDs emitting in visible spectral range can be found in several manufacturers' white papers, such as those from Cree, Nichia, and Osram.
Solid state (especially semiconductor) light sources, such as LEDs and laser diodes, are generally more compact in size, provide higher optical power, and have higher wall-plug efficiency when compared with lamps. For real space imaging applications, such as the light sources in data projectors, incoherent light generated LEDs are usually preferred over the coherent light generated by individual laser diodes, due to the image quality degrading effects (such as speckles) associated with coherent illumination. Moreover, the fabrication cost for LEDs is usually lower than that of laser diodes. Therefore, there are still many practical applications where LEDs are utilized instead of laser diodes. In terms of efficiency, however, within the realm of solid state light sources, lasers diodes are preferred over LEDs for the above-described applications. In particular, the internal quantum efficiencies, or the ratios between the net number of photons generated and the net number of electrical carrier (electron and holes) injected into a light emitting device, resulting from the stimulated emission process in laser diodes are seen to be more intrinsically efficient than the spontaneous emission process in LEDs. For the same reason, most LBPs employ laser diodes as their scanning light sources, rather than LEDs.
However, it should be noted that some laser diode arrangements are undesirable. For instance, intracavity-doubled diodes or frequency doubled DPSS (diode pumped solid state lasers) can be used to realize green/blue laser sources. Unlike semiconductor laser diodes, intracavity-doubled lasers require an extra step of converting optical energy from one wavelength to another wavelength. The frequency doubled DPSS designs require an even further step of optical pumping to the gain materials. These extra steps typically result in a significantly more complex device structure (high fabrication cost and larger device size) and lower wall-plug efficiencies.
As such, better solutions are seen to come from directly electrically injected laser diodes, which can operate at high optical powers. In data projector applications, each color light source needs output optical power in the range between several tens of milliwatts to several watts. Such power levels cannot typically be provided by existing microcavity laser designs, such as VCSELs (Vertical Cavity Surface Emitting Lasers). To reach such output power levels, a large gain volume is required, typically several hundred micrometers long semiconductor bars (Fabry-Perot) embedded with quantum wells (QWs). Compared to the wavelengths within visible and NIR (near infrared) light, a cavity this large (several hundred micrometers) is multimode within the >20 nm gain bandwidth provided by typical QW media.
A well controlled and stable operation of high power laser diodes is desired. Several approaches have been developed for realizing stable single mode operation (which is by definition coherent) at high optical power on semiconductor laser diode devices.
One such approach is the coherently coupled laser array, which is also referred to as the phase-locked laser array. There have been many designs, such as those described in U.S. Pat. No. 5,323,405, U.S. Pat. No. 5,365,541, Y. Liu, H. Liu, and Yehuda Braiman, Applied Optics 41(24), 5036-9 (2002).
Another approach to realizing stable single mode operation of high power laser diodes is the MOPA (master oscillator power amplifier), which has been described in studies such as J. N. Walpole, E. S. Kintzer, S. R. Chinn, C. A. Wang, L. J. Missaggia, “High-power strained-layer InGaAs/AlGaAs tapered traveling-waveamplifier”, Appl. Phys. Lett., 61 (1992) 740-742, and U.S. Pat. No. 6,721,344.
Yet another of such single mode laser diode approach is the Distributed Feedback (DFB) laser, which is used in telecommunications, and which has been described by patents within U.S. patent classifications 372, 92 and 96.
Most of these high power laser diodes have sophisticated designs and stable performances. However, such sophisticated systems are expensive to produce. For this reason, such lasers have not yet found their positions in the competitive consumer product markets of data projectors and LBPs.
In addition to the foregoing deficiencies, the coherent single wavelength property of the above-mentioned devices is not preferred for data projectors and LBPs applications. As noted above, a method to combine multiple light sources' output power incoherently is preferred, with such method resulting in a compact size, low cost, and stable design. By combining power incoherently, high total optical power can be realized, with each individual light source already providing high wall-plug efficiency.
Combining the output of multiple lasers incoherently has been addressed by some imaging systems. For example, a lens imaging system or an optical diffractive device can focus the output beams from multiple laser diodes into a common spot. Such a system has been described in U.S. Pat. No. 6,404,542 and Steven Serati, Hugh Masterson and Anna Linnenberger, “Beam combining using a Phased Array of Phased Arrays (PAPA)” 2004 IEEE Aerospace Conference, 5.0205 (March 2004).
However, such external optical lens systems (or extern diffraction systems) increase the size and cost of the light source. In addition, such systems have problems related to the free space alignment packaging costs, and the alignment stability susceptible operation environment. These systems are therefore not seen to combine the output power of multiple laser diodes in a monolithic integrated fashion, with reduced device size and fabrication costs, and with characteristics that match the preferred functional characteristics described above.
To achieve the functional characteristics described above, one may attempt to physically position an array of laser diodes near a waveguide (such as optical fiber, or other dielectric waveguides), so that the output optical powers from each laser diode can be combined in the waveguide and output together at one end of the waveguide. However, such a system will likely not function stably. In particular, the time reversibility of the electromagnetic filed propagation (governed by the Maxwell's equations) indicates that there will be strong cross-talk between each laser diode coupled to the common waveguide. Such cross-talk between lasers will simply tend to phase lock the lasers into an undesirable low loss supermode. An optical feedback and cross-talk system with multiple laser cavities and waveguides may support multiple supermodes with quality factors within the >20 nm gain bandwidth of the quantum wells, but such a system is likely to be unstable.
Another category of devices can be used to combine laser power incoherently. Such devices are used in WDM (wavelength division multiplex) systems implemented for telecommunication applications. A typical multiplexer in a WDM system uses optical grating (e.g. “WDM Technologies: Active Optical Components,” ed. A. K. Dutta et al., Academic Press (2002), pp. 52) and/or interferences (arrayed waveguide grating, i.e. AWG, available at http://www2.noah-c.com/english/Apollo/apss/awg/awg_e.htm) to combine signals from multiple wavelengths without cross-talk between each other.
Within various multiplexer designs, those with photonic crystal techniques (e.g. U.S. Pat. No. 6,738,551 and U.S. Patent Application Publication No. 20020191905) may be used, because of their compact sizes and monolithic fabrication compatibility. More importantly, if one can combine the passive multiplexer devices with active laser arrays (such as those described in U.S. Pat. No. 6,804,283) in a practical design, the desired functional characteristics mentioned above may be realized within a low fabrication cost package. However, certain considerations should be made when combining of passive multiplexer devices with active laser arrays.
One consideration is how to combine cavity-based light sources with a waveguide which is coupled to each of the light sources, within a substrate. Even if the power of the light source of each cavity is small, the individual powers can be accumulated in the waveguide.
Another consideration is how to pack a large amount (several dozens to several hundreds) of single mode light sources within close proximity of a waveguide, and still satisfy the features of efficient electrical injection and thermal dissipation. In addition, how to pack single mode light sources in close proximity, while avoiding optical and/or electrical cross-talk, should be considered.
Another consideration is how to deliver the total output power in a waveguide and still avoid overheating to the waveguide. In this regard, to realize the preferred functional characteristic of a well-defined and stable beam profile, a single mode waveguide is preferred. In this regard, the requirement for a single mode waveguide only requires that the waveguide be single mode through the wavelength range of the functioning devices; it does not require that the waveguide to be single mode in all wavelengths (i.e. 0 to infinity), or single mode in other ranges such as photonic crystal bandgap.
A single mode waveguide usually corresponds to a relatively small geometric cross-section. As such, delivering large optical power through the small waveguide cross-section is likely to result in device overheating and nonlinear optical effects caused by high optical power density.
In light of the above considerations, it is desired to combine the output optical power from multiple light sources in a common waveguide (bus line waveguide), while making sure there is no optical cross-talk between light source cavities through the bus line waveguide.